Abstract/Summary

Widely distributed proxy records indicate that
the Medieval Climate Anomaly (MCA; *900–1350 AD)
was characterized by coherent shifts in large-scale Northern
Hemisphere atmospheric circulation patterns. Although
cooler sea surface temperatures in the central and eastern
equatorial Pacific can explain some aspects of medieval
circulation changes, they are not sufficient to account for
other notable features, including widespread aridity
through the Eurasian sub-tropics, stronger winter westerlies
across the North Atlantic and Western Europe, and shifts in
monsoon rainfall patterns across Africa and South Asia.
We present results from a full-physics coupled climate
model showing that a slight warming of the tropical Indian
and western Pacific Oceans relative to the other tropical
ocean basins can induce a broad range of the medieval
circulation and climate changes indicated by proxy data,
including many of those not explained by a cooler tropical
Pacific alone. Important aspects of the results resemble
those from previous simulations examining the climatic
response to the rapid Indian Ocean warming during the late
twentieth century, and to results from climate warming
simulations—especially in indicating an expansion of the
Northern Hemisphere Hadley circulation. Notably, the
pattern of tropical Indo-Pacific sea surface temperature
(SST) change responsible for producing the proxy-model
similarity in our results agrees well with MCA-LIA SST
differences obtained in a recent proxy-based climate field
reconstruction. Though much remains unclear, our results
indicate that the MCA was characterized by an enhanced
zonal Indo-Pacific SST gradient with resulting changes in
Northern Hemisphere tropical and extra-tropical circulation
patterns and hydroclimate regimes, linkages that may
explain the coherent regional climate shifts indicated by
proxy records from across the planet. The findings provide
new perspectives on the nature and possible causes of the
MCA—a remarkable, yet incompletely understood episode
of Late Holocene climatic change.